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Abstract

We demonstrate optical modulation rates exceeding the conventional cavity linewidth limit using a silicon coupling modulated microring. Small-signal measurements show coupling modulation was free of the parasitic cavity linewidth limitations at rates at least 6× the cavity linewidth. Eye diagram measurements show coupling modulation achieved data rates > 2× the rate attainable by conventional intracavity phase modulation. We propose to use DC-balanced encoding to mitigate the inter-symbol interference in coupling modulation. Analysis shows that coupling modulation can be more efficient than intracavity modulation for large output swings and high-Q resonators. Coupling modulation enables very high-Q resonant modulators to be simultaneously low-power and high-speed, features which are mutually incompatible in typical resonant modulators studied to date.

Figures (7)

Schematics of (a) an intracavity modulated microring and (b) a coupling modulated microring that uses a 2 × 2 MZI-coupler as marked by the box. Optical microscope images of the fabricated SOI (c) microring with the 2 × 2 MZI-coupler marked by the box and (d) the reference MZI. The reference MZI was nominally identical to the MZI-coupler in the microring. The microring and MZI were separated by 620 μm on the die.

Measured transmission spectra for (a) tuning the coupling coefficient at a fixed resonance and (b) tuning the resonance wavelength with a fixed coupling coefficient. Independent tuning of the coupling and resonance wavelength using the thermal tuners was achieved.

(a) Electro-optic S21 measurements of the reference MZI, coupling modulation, and intracavity modulation. The RF cables, RF adapters, and bias tees have been de-embedded. (b) Optical small-signal modulation responses of coupling and intracavity modulation. Each curve is obtained by normalizing the electro-optic S21 of the microring to the S21 of the reference MZI and referencing to the value at 100 MHz. The microring was biased near critical coupling, with a cavity linewidth Δν ≈ 6 GHz. The intracavity modulation response for a ∼ 1.3 GHz detuning from resonance (blue) has a 3 dB bandwidth of 4.4 GHz, similar to the linewidth. A ∼ 5 GHz detuning produces a resonant sideband peak near the value of the detuning (red), and the 3 dB bandwidth is extended to ∼ 13 GHz. The coupling modulation response (black) does not roll-off to 40 GHz (more than 6× the linewidth).

(a) Intracavity modulation eye diagrams of an over-coupled microring (Δν ≈ 9 GHz). The eye opening is larger than in Fig. 4, confirming that the intracavity modulation bandwidth depends on the cavity linewidth. (b) Eye diagrams of the pre-emphasized electrical drive signals at 28 Gb/s (left) and the resultant optical output of the reference MZI (right). No remnants of the pre-emphasis are present in the optical output.